Characteristics And Relationships Between Precipitation And PM_2.5 In Ya’an City

Acid precipitation as well as PM_2.5pollution have become two major environmental problems in recent years,exploring their relationship is one of the hot spots of environmental research at home and abroad.From June 2017 to May 2018,32precipitation and 65 PM_2.5samples were synchronously collected in Yucheng,Ya’an.The p H value and conductivity in precipitation,water-soluble inorganic ions（WSIIs）in precipitation and PM_2.5were analyzed.The pollution characteristics and relationship between precipitation and PM_2.5,as well as source apportionment of ions were explored.The main results are as follows:（1）During the sampling period,the p H values of precipitation were in the range of3.74～6.43,with a volume-weighted mean（VWM）value of 5.15;the frequency of acid rain was about 46.8%;the conductivities were in the range of 12.74～255μs/cm,with a VWM value of 27.75μs/cm.The total concentrations of WSIIs in precipitation were in the range of 225.93～3234.04μeq/L,with a VWM value of 424.98μeq/L,their order of annual VWM concentration was:NH₄⁺>SO₄^2->NO₃^->Cl^->Ca²⁺>K⁺>Na⁺>F^->Mg²⁺.The sum of NH₄⁺,SO₄^2-and NO₃^-contributed for 66.2%of the total identified ions.Based on the backward trajectory model,WSIIs in precipitation were mainly affected by the crust source from the west-northwest direction,the anthropogenic source from the west-northwest and southwest directions,the ocean source from the east-southeast region.Based on the principal component analysis（PCA）,two sources were identified,F1 is about secondary formation among anthropogenic precursors and earth crust/construction dust,F2 is about long-transported sea-salt source or salt mine in Sichuan Basin,and biomass burning.（2）The concentrations of PM_2.5ranged from 8.93μg/m³to 121.55μg/m³,only two days of PM_2.5concentration exceeded the national ambient air quality standard of China.The average contribution of nine WSIIs to PM_2.5mass was 42.98%,and their concentrations were in the order of SO₄^2->NO₃^->NH₄⁺>Cl^->K⁺>Ca²⁺>Na⁺>F^->Mg²⁺.The sum of SO₄^2-,NO₃^-and NH₄⁺accounted for 89%of the total identified ions.Based on the PCA results,three sources were identified,F1 is secondary source related about agricultural or biomass burning,F2 is earth crust/construction dust,F3 is long-transported sea-salt source or salt mine in Sichuan Basin.（3）In general,the higher amount of precipitation,the higher removal of PM_2.5,the average removal percentage of PM_2.5was 26.1%.Due to different formation processes,removal results were different between primary ions(Cl^-、K⁺、Ca²⁺、Mg²⁺、F^-、Na⁺)and secondary ions(NH₄⁺、NO₃^-、SO₄^2-).The scavenging ratios（SR）of primary ions were in a higher range of 3775～7230,while SR of secondary ions were in a lower range of1696～2856.NO₃^-had higher contribution than that of SO₄^2-in precipitation,which was opposite the condition of PM_2.5,and contributions of primary ions in precipitation were higher than those in PM_2.5.The average NO₃^-/SO₄^2-ratio in PM_2.5and precipitation were0.66 and 1.14,respectively.The annual average acidity of PM_2.5in Yucheng,Ya’an,was2.40 neq/m³,its acidizing buffer capacity was-2.91 neq/m³.Although its removal percentage was not high,considering involving in the formation of cloud condensation nuclei,PM_2.5still has ability to influence the p H of precipitation,the higher acidity of PM_2.5,the lower p H of precipitation.